1. Field of the Invention
The invention disclosed in this specification relates to a structure of a thin film semiconductor device such as a thin film transistor or the like, and a method for fabricating the same.
2. Description of the Prior Art
There is known a structure for obtaining a display device which has a high display function by using a thin film transistor (TFT) in a liquid crystal display, the display substituting a cathode ray tube. This display is referred to as an active matrix type liquid crystal display device. This active matrix type liquid crystal display device is a display in which a thin film transistor is arranged in each of the pixel electrodes arranged in matrix to provide a high function display. To heighten the display function, the characteristics of the thin film transistor is required to be set to as high as possible.
The thin film transistor used in an active matrix type liquid crystal display device has a problem in that the thin film transistor is required to be formed on a glass substrate. In other words, to use the glass substrate as the substrate, there is a problem in that the substrate is limited in the fabrication process. Not only thin film transistors but also semiconductors needs to be heated to a high temperature (for example, of 800 to 1000xc2x0 C.) out of the necessity of diffusing impurity into silicon, activating impurity in silicon, and improving the crystallinity of silicon. However, the temperature that can be applied to the glass substrate is generally about 600xc2x0 C., and various new techniques are required to fabricate a high performance semiconductor device at a temperature level below this. For example, there are such techniques as a technique for irradiating an amorphous silicon film with laser light to crystallize the amorphous silicon film, and a technique for using the laser light irradiation for the diffusion and activation of the impurity. Since the technique for laser light irradiation causes an extremely small thermal damage to the glass substrate, this is an extremely useful technique when the low productivity is permitted.
FIG. 2 is a schematic sectional view of a conventionally known thin film transistor (generally referred to as TFT). What is shown in FIG. 2 is a thin film transistor which functions to prevent the intrusion of impurity into the active layer from the glass substrate. The active layer comprises a source area 203, a channel formation area 204 and a drain area 205. Then, as a gate insulating film 200, a silicon oxide film or a silicon nitride film is formed. A gate electrode 206 comprises a metal and semiconductors. Further, the whole element is covered with an interlayer insulating film 207 which comprises an appropriate insulator such as a silicon oxide film or the like. Further, a source electrode 208 is taken out from the source area 203 while a drain electrode 209 is taken out from the drain area 205.
The active layer comprising the source area 203, the drain area 205 and the channel formation area 204 is formed of crystalline silicon. As the crystalline silicon film, a silicon film formed of the amorphous silicon film crystallized by the laser light irradiation is used. However, there is no technique available for forming a single crystal silicon on the glass substrate. Although the film thus formed has crystallinity, the film has a quality in which a large amount of defects and levels are present. Although the film thus formed has crystallinity, the film has a quality in which defects and levels are present. To reduce the defects and levels in the silicon film, a method for neutralizing a dangling bond (unpaired connectors) of silicon which causes defects and levels by using a hydrogen atom. This holds true of a case in which the active layer is not crystalline silicon and is formed of amorphous silicon.
In this manner, in the silicon semiconductor film formed on the glass substrate, the silicon semiconductor film needs to contain hydrogen. However, when an attempt is made to cause the active layer formed of the silicon semiconductor to contain hydrogen, there is a problem in that hydrogen is diffused into the gate insulating film from the active layer.
On the other hand, in the structure shown in FIG. 2, it is not extremely favorable that mobile ions exist in the gate insulating film, the threshold value varies, and a hysterisis is generated in the C-V characteristics. Consequently, containing hydrogen in the active layer is a useful method on the one hand, it is a disadvantageous method on the other in that hydrogen is diffused in the gate insulating film.
The invention disclosed in this specification is intended to provide a structure of a semiconductor device wherein the active layer formed of a silicon semiconductor is allowed to contain hydrogen, and the hydrogen does not affect other areas and other parts.
A semiconductor device disclosed in this specification primarily comprises, an active layer formed of a silicon film, and a gate insulating film formed on the active layer, wherein a thin film represented by SiOxNy is formed between the aforementioned active layer and the aforementioned gate insulating film.
In the above structure, examples of the silicon film include an amorphous silicon film and a crystalline silicon film. Examples of the crystalline silicon film include a polycrystalline silicon film, a fine crystal silicon film, an amorphous silicon film partially including a crystal structure and a silicon film having a mixture of a crystal structure and an amorphous structure.
The active layer refers to a semiconductor layer constituting a thin film transistor. Generally the thin film transistor comprises a source/drain area with one conductivity-type and a channel formation area. Further, the active layer includes an offset gate area and a light dope area. When the crystalline silicon film is used, it is desirable that the density of hydrogen contained in the active layer is set to 0.001 to 5 atom %.
Further, in the aforementioned structure, either silicon nitride film or silicon oxide film may be adopted as a base film formed under an active layer. Further, as a base film, a thin film transistor represented by SiOxNy is further effectively used. A structure for substantially closing hydrogen in the active layer by substantially covering the active layer (in actuality a contact area for the source/drain area is present so that the active layer is not completely covered) with a thin film represented by SiOxNy formed as a base film.
Further, in the case where a crystalline silicon film is used which contains a metal element which promotes the crystallization of silicon as a silicon film which constitutes an active layer, it is useful to adopt the aforementioned structure. In other words, to form the crystalline silicon film formed by the metal element which promotes the crystallization into a semiconductor with higher electric properties, the aforementioned structure is adopted at the time of hydrogenation to enable further heightening the effect of the hydrogenation. Needless to say, this effect is extremely useful when hydrogen ions are actively contained in the active layer by hydrogen doping or the like.
Further, in the case where nickel is used as a metal element for promoting the aforementioned crystallization, the effect is even more conspicuous. Further an excess amount of the metal element for promoting the crystallization deteriorates the characteristics of semiconductors (which is approximate to the characteristics of the metal). Excessively small amount of the metal element reduces the effect of promoting the crystallization. Consequently, the most appropriate density is 1xc3x971015 to 1xc3x971019 cmxe2x88x923.
As metal elements for promoting the crystallization, such elements as Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au can be used in addition to Ni. What is important about this element for promoting the crystallization of the amorphous silicon is that the element is an intrusive atom.
The metal element described above diffuses into the silicon film in the heat treatment step. Then the crystallization of silicon proceeds at the same time when the aforementioned element diffuses. In other words, the aforementioned intrusive metal exhibits catalytic action in various places of diffusion to promote the crystallization of the amorphous silicon film.
Besides, since the aforementioned intrusive element soon diffuse into the silicon film, the introduced amount of the element (added amount) becomes very important. In other words, when the introduced amount thereof is small, a favorable crystallinity cannot be obtained. Further, when the introduced amount is too large, the semiconductor characteristics of silicon will be lost.
Consequently, the most appropriate scope of the introduced amount of the aforementioned metal element into the amorphous silicon film becomes important. For example, when nickel is used as a metal element for promoting the aforementioned crystallization, the effect of promoting crystallization can be obtained by introducing nickel element into the amorphous silicon so that the density of the metal element in the crystalline silicon film becomes 1xc3x971019 cmxe2x88x923 or more. Further, it has been made clear that the semiconductor characteristics are not damaged when the introduced amount of nickel element is controlled so that the density of nickel element becomes 1xc3x971019 cmxe2x88x923 or less. The density here is defined by the minimum value obtained by the secondary ion mass analysis (SIMS) process.
Further, with respect to a metal element other than the aforementioned nickel, the effect can be obtained in the density scope similar to that of nickel.
In addition to the aforementioned metal element, when Al or Sn is used, the crystallization of the amorphous silicon film can be promoted. However, Al and Sn form an alloy with silicon and will not diffuse into and intrude the silicon film. In such a case, a portion where an alloy is formed with silicon in crystallization constitutes a crystal nucleus, and the crystal growth proceeds from that portion. When Al and Sn are used in this manner, the crystal growth proceeds only from a portion into which Al and Sn are introduced (an alloy layer of these elements and silicon). Consequently, there is a problem in that the crystallinity is generally poor as compared with a case in which an intrusive element such as the aforementioned nickel or the like is used. For example, there is a problem in that it is difficult to obtain a uniformly crystallized crystalline silicon film. Further, there is a problem in that the presence of the alloy layer hinders the fabrication of the device. Further, there is a problem in that the presence of the alloy layer deteriorates the reliability of the device.
With respect to the thin film represented by SiOxNy, X and y assume a value of 0 less than x less than 2 and 0 less than y less than 4/3 respectively, the dielectric constant assumes 4 to 6, and the band gap assumes 5.3 to 7.0 eV. The thin film represented by SiOxNy can be formed by using dichlorosilane (SiH2Cl2) or ammonia (NH4) and nitrogen monoxide (N2O). In this state, the thin film of SiOxNy includes chlorine at a concentration of 1xc3x971015 to 1xc3x971020 cmxe2x88x923.
A main structure of another aspect is characterized by comprising the steps of, forming an active layer comprising a silicon semiconductor on a substrate having an insulating surface, forming a thin film represented by SiOxNy by covering the aforementioned active layer, and forming a gate insulating film represented by the aforementioned SiOxNy.
In the aforementioned structure, examples of substrates having an insulating surface include a glass substrate, a semiconductor substrate on which an insulating film is formed, and a conductive substrate on which the insulating film is formed.
In the aforementioned structure, the step of forming the silicon semiconductor includes a method for forming an amorphous silicon film by the plasma CVD, the reduced pressure CVD, a method of crystallizing an amorphous silicon film formed by the plasma CVD and the reduced pressure CVD by the laser light irradiation or by heat treatment, and a method of crystallizing an amorphous silicon film formed by the plasma CVD or the reduced pressure CVD by the action of elements which promotes the crystallization of nickel or the like.
A structure of another aspect is characterized by comprising the steps of, forming an active layer comprising a silicon semiconductor on a substrate having an insulating surface, allowing the aforementioned active layer to contain hydrogen, forming a thin film represented by SiOxNy by covering the aforementioned active layer, and forming a gate insulating film comprising a silicon oxide film on a thin film represented by the aforementioned SiOxNy.
In the aforementioned structure, a method of allowing an active layer to contain hydrogen includes such methods as hydrogen ion doping, heat treatment in the atmosphere of hydrogen, and exposure to hydrogen plasma.
In this structure, too, it is useful to use a crystalline silicon film by using a catalyst element which promotes crystallization.
A silicon semiconductor which contains hydrogen or which is allowed to contain hydrogen is used as an active layer. In a structure in which a gate insulating film is present on the active layer, hydrogen in the active layer does not diffuse into the active layer by forming a thin film represented by SiOxNy between the active layer and the gate insulating film. Then, a thin film transistor having an excellent electric characteristics and stability can be obtained. Further, a thin film transistor represented by SiOxNy is formed by using a chlorosilane and dichlorosilane with the result that a film is allowed to contain chlorine. This chlorine serves to fix mobile ions to heighten the function and stability as a gate insulating film.
Further, by using a thin film represented by SiOxNy a structure can be realized wherein an active layer is substantially covered by a thin film represented by SiOxNy. Then, hydrogen which is contained in the active layer can be closed in the active layer thereby heightening the effect. Further, at the same time, hydrogen in the active layer can be prevented from diffusing to the outside of the active layer.
A thin film represented by SiOxNy not only has a barrier effect with respect to hydrogen ions but also oxygen (O) in the film serves to remove hysterisis in the C-V characteristics. Further, the SiN bond serves to prevent the drift of Na and heavy metals (such as Fe, Ni and Co).
In particular, when the active layer is crystallized by using a metal element such as nickel or the like, the metal element is contained in the active layer. Consequently, it is extremely useful to cover at least an upper surface of the active layer (a surface which contacts the gate insulating film) with a thin film represented by SiOxNy. In other words, metal elements such as nickel or the like which functions as a mobile ion can be prevented from diffusing into the gate insulating film.